fwrite respects dec=',' for time columns when needed

Author: MichaelChirico

NEWS.md

@@ -10,6 +10,8 @@
 
 1. Using `print.data.table()` with character truncation using `datatable.prettyprint.char` no longer errors with `NA` entries, [#6441](https://github.com/Rdatatable/data.table/issues/6441). Thanks to @r2evans for the bug report, and @joshhwuu for the fix.
 
+2. `fwrite()` respects `dec=','` for timestamp columns (`POSIXct` or `nanotime`) with sub-second accuracy, [#6446](https://github.com/Rdatatable/data.table/issues/6446). Thanks @kav2k for pointing out the inconsistency and @MichaelChirico for the PR.
+
 ## NOTES
 
 1. Tests run again when some Suggests packages are missing, [#6411](https://github.com/Rdatatable/data.table/issues/6411). Thanks @aadler for the note and @MichaelChirico for the fix.

inst/tests/other.Rraw

@@ -761,3 +761,8 @@ if (loaded[["dplyr"]]) {
   DT = data.table(a = 1, b = 2, c = '1,2,3,4', d = 4)
   test(30, DT[, c := strsplit(c, ',', fixed = TRUE) %>% lapply(as.integer) %>% as.list]$c, list(1:4)) # nolint: pipe_call_linter. Mimicking MRE as filed.
 }
+
+if (loaded[["nanotime"]]) {
+  # respect dec=',' for nanotime, related to #6446, corresponding to tests 2281.*
+  test(31, fwrite(data.table(as.nanotime(.POSIXct(0))), dec=',', sep=';'), output="1970-01-01T00:00:00,000000000Z")
+}

inst/tests/tests.Rraw

@@ -19059,3 +19059,7 @@ test(2280.1, internal_error("broken"), error="Internal error.*broken")
 test(2280.2, internal_error("broken %d%s", 1, "2"), error="Internal error.*broken 12")
 foo = function(...) internal_error("broken")
 test(2280.3, foo(), error="Internal error in foo: broken")
+
+# fwrite respects dec=',' for sub-second timestamps, #6446
+test(2281.1, fwrite(data.table(a=.POSIXct(0.001)), dec=',', sep=';'), output="1970-01-01T00:00:00,001Z")
+test(2281.2, fwrite(data.table(a=.POSIXct(0.0001)), dec=',', sep=';'), output="1970-01-01T00:00:00,000100Z")

src/fwrite.c

@@ -438,19 +438,21 @@ void writePOSIXct(const void *col, int64_t row, char **pch)
     ch -= squashDateTime;
     write_time(t, &ch);
     if (squashDateTime || (m && m%1000==0)) {
+      Rprintf("here i\n");
       // when squashDateTime always write 3 digits of milliseconds even if 000, for consistent scale of squash integer64
       // don't use writeInteger() because it doesn't 0 pad which we need here
       // integer64 is big enough for squash with milli but not micro; trunc (not round) micro when squash
       m /= 1000;
-      *ch++ = '.';
+      *ch++ = dec;
       ch -= squashDateTime;
       *(ch+2) = '0'+m%10; m/=10;
       *(ch+1) = '0'+m%10; m/=10;
       *ch     = '0'+m;
       ch += 3;
     } else if (m) {
+      Rprintf("here ii\n");
       // microseconds are present and !squashDateTime
-      *ch++ = '.';
+      *ch++ = dec;
       *(ch+5) = '0'+m%10; m/=10;
       *(ch+4) = '0'+m%10; m/=10;
       *(ch+3) = '0'+m%10; m/=10;
@@ -472,6 +474,7 @@ void writeNanotime(const void *col, int64_t row, char **pch)
   if (x == INT64_MIN) {
     write_chars(na, &ch);
   } else {
+    Rprintf("here iii\n");
     int d/*days*/, s/*secs*/, n/*nanos*/;
     n = x % 1000000000;
     x /= 1000000000;
@@ -488,7 +491,7 @@ void writeNanotime(const void *col, int64_t row, char **pch)
     *ch++ = 'T';
     ch -= squashDateTime;
     write_time(s, &ch);
-    *ch++ = '.';
+    *ch++ = dec;
     ch -= squashDateTime;
     for (int i=8; i>=0; i--) { *(ch+i) = '0'+n%10; n/=10; }  // always 9 digits for nanoseconds
     ch += 9;